1. Field of the Invention
The present invention relates to an apparatus for controlling that supply of hydraulic pressure to a hydraulic controller of an automatic transmission in a vehicle, such as an automobile. Further, the present invention is especially suitable for a hybrid vehicle, and a vehicle performing an idling stop, or the like. More particularly, the present invention relates to an apparatus which controls operating voltage of an electric oil pump which supplies a hydraulic pressure to a hydraulic controller of the automatic transmission, based on oil temperature, when the drive power source of the vehicle is stopped (off).
2. Description of Related Art
Some vehicles such as hybrid vehicles and vehicles which come to an idling stop, automatically stop (turn off) their drive power source (such as an engine and a motor) when the vehicle is stopped (or when predetermined conditions are established) in order to reduce exhaust emissions and improve fuel economy. In these vehicles, a mechanical oil pump for supplying a hydraulic pressure to the hydraulic controller of the automatic transmission, or an automatic transmission mechanism or the like, is mechanically interlocked with the drive power source, and therefore it is stopped when the drive power source is inoperative. When this happens, the hydraulic pressure decreases below that required for controlling clutches to be used for transmitting a drive force. Therefore, when the drive power source is restarted, the clutches are engaged only after the rotational speed of the power source has increased to a level where a shock is generated.
Japanese Patent Application Laid-Open No. 8-14076, for example, discloses a system wherein an electric oil pump is electrically driven by a battery or the like, independently of the drive power source. The system is designed such that, when the mechanical oil pump is stopped, the electric oil pump, which is independent of the drive power source, is driven so as to supply a hydraulic pressure to a hydraulic controller, so that a predetermined hydraulic pressure required for hydraulic control is maintained.
However, as disclosed in the aforementioned patent application, the electric motor for driving the electric oil pump is always operated under a constant voltage. Therefore, a greater hydraulic pressure than is required is generated at some oil temperatures, because of the characteristics of the automatic transmission, change in oil viscosity due to oil temperature or the like. This causes an increase in load on the electric oil pump and the electric motor, thereby increasing the electric power consumption of the electric motor and decreasing the amount of charge of the battery, which may lead to a decrease in operation time of the electric motor. In addition, durability of the electric oil pump and the electric motor is reduced. Furthermore, the electric oil pump must be designed to withstand a greater hydraulic pressure than is required as described above.
Therefore, it is an object of the present invention to provide a controller for, based on an oil temperature, supplying a determined operating voltage to an electric motor for driving an oil pump, so as to solve the foregoing problems.